Fixes for CentOS 6/7.

CentOS 6 and 7 have different paths where mailcatcher is
installed.
Updated dependencies version numbers.
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 which
is not available in CentOS6.
Maybe you need the epel repository so that these packages get
installed.
Raised stdlib requirement to >= 4.2.0 for union().

For merging package arrays I use union() which was added to stdlib
in 4.2.0.
Add rubygem-mime-types package from EPEL.

current mime-types from gem raised requirements and need ruby >= 1.9.2
which is not available on CentOS6.
On CentOS7 the gem installed mime-types 2.3 throws
"Encoding::CompatibilityError"s.
So in both cases use the package from EPEL which just works fine for
CentOS6 and CentOS7.
[BUGFIX] gem installed multi_json causes mailcatcher crash.

In some places I encountered a crash of mailcatcher when it receives a mail
caused by the multi_json version installed through gem.
Switch to EPEL package. I don't understand this whole ruby gem dependency and
version madness.
Merge pull request #16 from hdi-kw/master

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