Break .travis.yml to two files

* .travis.yml = the primary Travis config file.
* .travis.runonce.bash = the script which we want to run only once per
build.
.travis.runonce.bash: add forgotten sphinx
.travis.runonce.bash: add msgcheck

&& requirements.txt: add msgcheck & forgotten sphinx.
fix .travis.yml
Merge pull request #765 from Mkaysi/runonce

Break .travis.yml to two files