Another try to get some problems fixed for CentOS 6 and CentOS7.

CentOS 6 and 7 have different paths where mailcatcher is installed.
Add rubygem-json_pure as it seems to be a runtime dependency for mailcatcher in CentOS7 which is not installed by default.
Add rubygem-mime-types as it requires ruby >= 1.9.2 now when installed through gem which is not available in CentOS6.
Maybe you need the epel repository so that these packages get installed from there.
Updated dependencies version numbers in Modulefile and README.md.
I still have problems on older CentOS6 boxes, but this module installs and runs mailcatcher now on up to date CentOS 6.5 and CentOS 7 with EPEL repository added. Please test and report if you find other issues.