Update Rack (2.0.8 -> 2.2.2)

Thanks to rack/rack#1545, no more warnings about
the magic comment

**Before**

    $ bundle e rake
    Run options: --seed 50920

    # Running:

    .................

    Finished in 0.046828s, 363.0307 runs/s, 405.7402 assertions/s.

    17 runs, 19 assertions, 0 failures, 0 errors, 0 skips
    config.ru:1: warning: `frozen_string_literal' is ignored after any tokens
    I, [2020-04-19T19:51:59.845036 #67031]  INFO -- sentry: ** [Raven] Raven 2.9.0 configured not to capture errors: DSN not set
    config.ru:1: warning: `frozen_string_literal' is ignored after any tokens
    Run options: --seed 40615

    # Running:

    ..........

    Finished in 1.222135s, 8.1824 runs/s, 10.6371 assertions/s.

    10 runs, 13 assertions, 0 failures, 0 errors, 0 skips
    Running RuboCop...
    Inspecting 48 files
    ................................................

    48 files inspected, no offenses detected

**After**

    $ bundle e rake
    Run options: --seed 63735

    # Running:

    .................

    Finished in 0.021696s, 783.5546 runs/s, 875.7375 assertions/s.

    17 runs, 19 assertions, 0 failures, 0 errors, 0 skips
    I, [2020-04-19T19:57:25.660098 #68534]  INFO -- sentry: ** [Raven] Raven 2.9.0 configured not to capture errors: DSN not set
    Run options: --seed 43133

    # Running:

    ..........

    Finished in 1.161220s, 8.6116 runs/s, 11.1951 assertions/s.

    10 runs, 13 assertions, 0 failures, 0 errors, 0 skips
    Running RuboCop...
    Inspecting 48 files
    ................................................

    48 files inspected, no offenses detected

Gemfile.lock

@@ -57,7 +57,7 @@ GEM
     public_suffix (3.1.0)
     puma (4.3.3)
       nio4r (~> 2.0)
-    rack (2.0.8)
+    rack (2.2.2)
     rack-flash3 (1.0.5)
       rack
     rack-protection (2.0.5)