Skip loading RMagick if already loaded

[mshibuya]

In a case-insensitive filesystem environment, there is a possibility that RMagick gets loaded twice(require 'rmagick' and require 'RMagick' loads the same file, but Ruby sees them as different ones).
This PR solves it by loading RMagick only if needed.

Refs #1205, #1330

[Bodacious]

+1

[davidbegin]

+1 this fixed the double load problem I was having.

[jaredbeck]

It looks like this hasn't been released yet?

git describe --exact-match c0bb87f2ab9b4f1ae94cc622923567690c70c840
fatal: no tag exactly matches 'c0bb87f2ab9b4f1ae94cc622923567690c70c840'

Is there an ETA on the next release? Thanks @mshibuya @bensie for fixing it.

[jaredbeck]

One week ping for @bensie: can we get a release with this fix? Maybe 0.10.1?

[bensie]

@jaredbeck Please just use master from Bundler. We'll get a release out as soon as we can.

[jaredbeck]

Please just use master from Bundler. We'll get a release out as soon as we can.

Will do. Thanks!