Update README.md

fix g++ version on OSX
[skip ci]
Fix OSX linker warning issue
Put MemCheck and Submit to "after_script" section to ignore possible failures
Put LEANREPO variable in .travis.yml to check the blessed repository inside of travis-ci.

It prevents other users from running heavy tasks on travis and submitting results to cdash server.
Put LEANREPO variable in .travis.yml to check the blessed repository inside of travis-ci.