suppress_warning for ENV#index specs

* Which is more reliable than filtering the warnings as the message or
  caller might differ.

Author: eregon

lib/mspec/utils/warnings.rb

@@ -45,7 +45,6 @@ def Warning.warn(message)
     when /\/(argf|io|stringio)\/.+(ARGF|IO)#(lines|chars|bytes|codepoints) is deprecated/
     when /Thread\.exclusive is deprecated.+\n.+thread\/exclusive_spec\.rb/
     when /hash\/shared\/index\.rb:\d+: warning: Hash#index is deprecated; use Hash#key/
-    when /env\/shared\/key\.rb:\d+: warning: ENV\.index is deprecated; use ENV\.key/
     when /exponent(_spec)?\.rb:\d+: warning: in a\*\*b, b may be too big/
     when /enumerator\/(new_spec|initialize_spec)\.rb:\d+: warning: Enumerator\.new without a block is deprecated/
     when /Pattern matching is experimental, and the behavior may change in future versions of Ruby!/