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

CentOS 6 and 7 have different paths where mailcatcher is installed. Fixed that.
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. On CentOS7 the gem installed mime-types causes encoding problems. So use the EPEL package.
Add rubygem-multi_json from EPEL as the gem installed one causes mailcatcher crashes when receiving mails.
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 enabled. Please test and report if you find other issues.