Bump version number of cabal-install to 1.18.0.1
Catch exceptions in findUsableBranchers

In the version of process that comes with GHC 7.7,
readProcessWithExitCode raises an exception when
the process could not be started (for example if
the command to be run doesn't exist).
Eliminate a warning in Distribution.Client.Get

System.Cmd has simply reexported rawSystem from System.Process
since at least process-1.0.1.1, the minimum version we allow,
and has been deprecated just as long :)
Merge pull request #1491 from rwbarton/rwbarton-get-s

"cabal get --source-repository" fails under ghc 7.7 when a brancher is missing
Merge pull request #1492 from rwbarton/rwbarton-warning

Eliminate a warning in Distribution.Client.Get
Unbreak the build on GHC <= 7.4.

Fixes #1494.