Change doxygen download URL for Travis

[danpoe]

The previous download URL doesn't work anymore

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[NlightNFotis]

Thanks Dan!

[hannes-steffenhagen-diffblue]

We had some concerns about this maybe breaking the doxygen checks on travis, let's wait and see if those work before merging.

[thk123]

I'm happy if it works - though should we switch back (and update the version), that way we're using the latest version

[danpoe]

@hannes-steffenhagen-diffblue This is the same version as we had previously, so all the checks should still pass.

@thk123 Just wanted to get this in quickly so merges on develop aren't blocked. We can later see about switching to the latest version of doxygen. The newest version is also available via the source forge link so we'd just need to change DOXYGEN_VERSION in the travis.yml file.

[codecov-io]

Codecov Report

Merging #5329 into develop will decrease coverage by 1.54%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #5329      +/-   ##
===========================================
- Coverage    68.05%   66.51%   -1.55%     
===========================================
  Files         1170     1170              
  Lines        96421    96421              
===========================================
- Hits         65620    64134    -1486     
- Misses       30801    32287    +1486     

Flag	Coverage Δ
#cproversmt2	?
#regression	64.55% <ø> (ø)
#unit	31.77% <ø> (ø)

Impacted Files	Coverage Δ
src/solvers/smt2/letify.cpp	0.00% <0.00%> (-100.00%)	⬇️
src/solvers/smt2/smt2_dec.h	0.00% <0.00%> (-100.00%)	⬇️
src/solvers/floatbv/float_bv.h	0.00% <0.00%> (-100.00%)	⬇️
src/solvers/smt2/smt2irep.cpp	0.00% <0.00%> (-88.00%)	⬇️
src/solvers/floatbv/float_bv.cpp	0.00% <0.00%> (-74.13%)	⬇️
src/solvers/smt2/smt2_dec.cpp	0.00% <0.00%> (-67.70%)	⬇️
src/solvers/smt2/letify.h	33.33% <0.00%> (-66.67%)	⬇️
src/solvers/smt2/smt2_conv.h	36.36% <0.00%> (-63.64%)	⬇️
src/solvers/smt2/smt2_conv.cpp	19.81% <0.00%> (-37.61%)	⬇️
src/solvers/flattening/pointer_logic.cpp	71.21% <0.00%> (-18.19%)	⬇️
... and 17 more

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