Update rack gem

There were two CVEs filed against the rack gem, this bumps the version
from 2.0.5 to 2.0.6.

[CVE-2018-16471][1]

> moderate severity
> Vulnerable versions: >= 2.0.0, < 2.0.6
> Patched version: 2.0.6
>
> There is a possible XSS vulnerability in Rack before 2.0.6 and 1.6.11.
> Carefully crafted requests can impact the data returned by the scheme
> method on Rack::Request. Applications that expect the scheme to be
> limited to 'http' or 'https' and do not escape the return value could be
> vulnerable to an XSS attack. Note that applications using the normal
> escaping mechanisms provided by Rails may not impacted, but applications
> that bypass the escaping mechanisms, or do not use them may be
> vulnerable.

[CVE-2018-16470][2]

>  moderate severity
>  Vulnerable versions: >= 2.0.4, < 2.0.6
>  Patched version: 2.0.6
>
>  There is a possible DoS vulnerability in the multipart parser in Rack
>  before 2.0.6. Specially crafted requests can cause the multipart parser
>  to enter a pathological state, causing the parser to use CPU resources
>  disproportionate to the request size.

[1]: https://nvd.nist.gov/vuln/detail/CVE-2018-16471
[2]: https://nvd.nist.gov/vuln/detail/CVE-2018-16470

rails/Gemfile.lock

@@ -169,7 +169,7 @@ GEM
     puma (3.12.0)
     pundit (2.0.0)
       activesupport (>= 3.0.0)
-    rack (2.0.5)
+    rack (2.0.6)
     rack-proxy (0.6.5)
       rack
     rack-test (1.1.0)