Display python version & platform at repl startup

*hy/cmdline.py: At the repl startup the Python implementation & version
 is displayed along with the platform(os) information
Make ci builds faster by splitting requirements

Make travis builds faster by splitting requirements into
requirements-travis.txt, which contains all the requirements necessary
for running tests, documentation related packages are omitted and added
as a part of requirements-dev.txt

Additionally .travis.yml is modified to cache pip downloads, though this
feature is yet to be rolled out for the free tier
Merge branch 'master' into pr/585
Merge branch 'master' into pr/599
Rephrased confusing comment
Merge pull request #603 from Duta/patch-1

Rephrased confusing comment
Minor typo

Sorry for tiny commits :P
Merge pull request #604 from Duta/master

Minor typo
docs: fix decorator to reflect multiple decorators

* docs/language/api.rst: the `with-decorator' builtin supports multiple
  decorators which it applies in order. Docs are updated to reflect this
Remove HyLambdaListKeyword from the parser

It's not a syntactic element and doesn't belong in the parser. Parsing
lambda lists is now handled by the compiler alone.
Merge branch 'master' into pr/611