Remove HTTP repo work-around for JDK6

[ashawley]

We changed artifact retrieval to HTTP in #247 for the JDK6 build on Travis. We're probably going to drop 2.11 support in #280, which will make the JDK6 unnecessary. The HTTP part of the build for JDK6 can be ripped out and simplified.

The only release made that used this method was Scala XML 1.1.1 for Scala 2.11 (including the Scala.js 0.6 artifact for 2.11).

Publishing 2.11 builds from JDK6 used HTTPS to bintray, so there were no credentials exposed.

All the other builds are unaffected. They were built on JDK8 and used HTTPS.