Allow minor version upgrades for package requirements (#422)

* Allow minor version upgrades for package requirements

According to [PEP440](https://www.python.org/dev/peps/pep-0440/#compatible-release),
`~=` operator will do setup a prefix matching of the last version number. So
`pkg~=1.1.0` will allow only patch version upgrades, while `pkg~=1.1` will
allow all minor version upgrades while preventing major version changes ie. 2.x

This was my intention with an earlier commit but unfortunately I missed the
fine print in PEP440. This is especially important for boto3 because of how
often they release. Users that have any version of v1.5<=boto3<v2.0 should
be compatible with SAM Translator. SAM Translator uses boto3 only to query
the default region configured in their shell. Being compatible with patch
and minor versions allows SAM Translator to be installable in computers
that have any relatively recent (> Dec 2017) version of AWS CLI. Without
this change, SAM Translator cannot be run on computers with a more recent
version of AWS CLI installation that bundles a recent version of boto3.

This is a good practice in general to allow for easy deployment of security
changes as recommended by [PEP404](https://www.python.org/dev/peps/pep-0440/#version-matching)

* Bumping to v1.5.4 to prep for release

Author: sanathkr

requirements/base.txt

@@ -1,5 +1,5 @@
-boto3~=1.5.0
-enum34~=1.1.6
-jsonschema~=2.6.0
-six~=1.11.0
+boto3~=1.5
+enum34~=1.1
+jsonschema~=2.6
+six~=1.11
 

samtranslator/__init__.py

@@ -1 +1 @@
-__version__ = '1.5.3'
+__version__ = '1.5.4'