Bump rack from 1.4.1 to 2.0.8 in /edx_specs

Bumps [rack](https://github.com/rack/rack) from 1.4.1 to 2.0.8.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@1.4.1...2.0.8)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

edx_specs/Gemfile.lock

@@ -43,7 +43,7 @@ GEM
       coderay (~> 1.0.5)
       method_source (~> 0.8)
       slop (~> 3.3.1)
-    rack (1.4.1)
+    rack (2.0.8)
     rack-test (0.6.1)
       rack (>= 1.0)
     rspec (2.7.0)