Deactivate isolation spec under non MRI-2.1.2

The specs check for event sequences that are not consistently
triggerable cross MRI versions. The semantics work under all versions.
Nuke UTF-8 encoding headers

* I do not use 1.9.3
* Also keeping them in each file increases mental overhead (true it *can* be autoamted)
* None of the files encodes NON ASCII chars.
* I do not expect it makes any difference, since nobody programmatically
  will consume strings generated by mutant under the assumption they are UTF-8 encoded.
* 1.9.3 Users have to deal with the encoding fuckup under ruby anyways.