Enable rustfmt-nightly to pass the filter

rustfmt-nightly was forked from rustfmt 0.9 and its
version numbering has been reset to 0.1.0.
This commit updates the version check to reject rustfmt only
if its 0.[1-4].x version is not follwed by '-nightly' suffix.

CHANGELOG.md

@@ -1,5 +1,6 @@
 # Next
 - Fixes #1638. update type-formatter and Fixes indent size not supported
+- Add support for rustfmt-nightly
 
 # v0.30.4 (2017-07-14)
 - Fixes #1732. Improve deprecation message for old options for new Executables

src/beautifiers/rustfmt.coffee

@@ -34,7 +34,7 @@ module.exports = class Rustfmt extends Beautifier
     else
       @run(program, ["--version"], help: help)
         .then((stdout) ->
-          if /^0\.(?:[0-4]\.[0-9])/.test(stdout.trim())
+          if /^0\.(?:[0-4]\.[0-9])(?!-nightly)/.test(stdout.trim())
             versionCheckState = false
             throw new Error("rustfmt version 0.5.0 or newer required")
           else