Fix link to COMPOSER_HOME
Merge pull request #2323 from yuya-takeyama/patch-1

Fix link to COMPOSER_HOME on doc
Allow specifying HHVM as a dependency
hhvm version constant still called HPHP on older versions so use it for bc
Check only part of the error message as it's different in hhvm
Fix stream context option test to really only verify content-type is last