Relaxing ruby 2.5 version and silencing RVM warning

[dgilperez]

This PR makes it possible to cope with Ruby 2.5 patch versions (2.5.5 for instance).

It also moves from .rvmrc to newer .ruby-version and .ruby-gemset as suggested by RVM itself:

You are using '.rvmrc', it requires trusting, it is slower and it is not compatible with other ruby managers,
you can switch to '.ruby-version' using 'rvm rvmrc to ruby-version'
or ignore this warning with 'rvm rvmrc warning ignore /Users/dgilperez/src/i4a/seal/.rvmrc',
'.rvmrc' will continue to be the default project file in RVM 1 and RVM 2,
to ignore the warning for all files run 'rvm rvmrc warning ignore all.rvmrcs'.

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• NOTICE *

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• RVM has encountered a new or modified .rvmrc file in the current directory, this is a shell script and *
• therefore may contain any shell commands. *

•                                                                                                             *
  

• Examine the contents of this file carefully to be sure the contents are safe before trusting it! *
• Do you wish to trust '/Users/dgilperez/src/i4a/seal/.rvmrc'? *
• Choose v[iew] below to view the contents *

[binaryberry]

Thanks for the contribution David!