Mention 'subversion' in error message when svn is missing

Since the svn binary is brought by the "subversion" package in most systems, we should prompt the user to install the subversion package. Nonetheless there may be users of ruby-build who are unaware of svn being a command of the subversion scm.
How about saying "Please install Subversion and try again?" Capitalized name, no backticks. This is because `` `subversion` `` is not an executable name on the command line.