Fix back compatiblity

We need to make sure these strings are muable, but `String#+@` is only
available in Ruby 2.3+ and the gem doesn't yet restrict the Ruby version
we're compatible with.

Fix backwards compatiblity by releasing a version using `String#dup`,
then we can add the Ruby version restriction in the Gemfile and revert
to `String#+@`.

lib/rspec_junit_formatter.rb

@@ -93,7 +93,7 @@ def xml_dump_output(example)
 
   # Inversion of character range from https://www.w3.org/TR/xml/#charsets
   ILLEGAL_REGEXP = Regexp.new(
-    +"[^" <<
+    "[^".dup <<
     "\u{9}" << # => \t
     "\u{a}" << # => \n
     "\u{d}" << # => \r
@@ -125,7 +125,7 @@ def xml_dump_output(example)
   # Discouraged characters from https://www.w3.org/TR/xml/#charsets
   # Plus special characters with well-known entity replacements
   DISCOURAGED_REGEXP = Regexp.new(
-    +"[" <<
+    "[".dup <<
     "\u{22}" << # => "
     "\u{26}" << # => &
     "\u{27}" << # => '