Change Error message for missing subversion

The current error message for missing subversion is "error: please install \`svn\` and try again", leading the user to try to find and install a package named "svn" (eg. `aptitude install svn for # debian / ubuntu). Changing it to "subversion" can help users unaware that svn is a binary installed by the subversion package, get through the error.
Merge pull request #669 from Zorbash/master

Mention 'subversion' in error message when svn is missing