LIBHYDRA-105 Updated "sass-rails" gem to remove deprecation warnings

Update "sass-rails" gem from v5.0.5 to v5.0.6 to fix the following
deprecation warnings appearing at startup and running tests:

```
DEPRECATION WARNING: Sprockets method `register_engine` is deprecated.
Please register a mime type using `register_mime_type` then
use `register_compressor` or `register_transformer`.
https://github.com/rails/sprockets/blob/master/guides/extending_sprockets.md#supporting-all-versions-of-sprockets-in-processors
 (called from block (2 levels) in <class:Railtie> at /Users/dsteelma/.rvm/gems/ruby-2.2.4@archelon/gems/sass-rails-5.0.5/lib/sass/rails/railtie.rb:57)
DEPRECATION WARNING: Sprockets method `register_engine` is deprecated.
Please register a mime type using `register_mime_type` then
use `register_compressor` or `register_transformer`.
https://github.com/rails/sprockets/blob/master/guides/extending_sprockets.md#supporting-all-versions-of-sprockets-in-processors
 (called from block (2 levels) in <class:Railtie> at /Users/dsteelma/.rvm/gems/ruby-2.2.4@archelon/gems/sass-rails-5.0.5/lib/sass/rails/railtie.rb:58)
```

See rails/sass-rails#381

https://issues.umd.edu/browse/LIBHYDRA-105

Gemfile

@@ -5,7 +5,7 @@ gem 'rails', '4.2.6'
 # Use sqlite3 as the database for Active Record
 gem 'sqlite3'
 # Use SCSS for stylesheets
-gem 'sass-rails', '~> 5.0'
+gem 'sass-rails', '~> 5.0', '>= 5.0.6'
 # Use Uglifier as compressor for JavaScript assets
 gem 'uglifier', '>= 1.3.0'
 # Use CoffeeScript for .coffee assets and views

Gemfile.lock

@@ -174,7 +174,7 @@ GEM
     ruby-progressbar (1.8.1)
     rubyzip (1.2.0)
     sass (3.4.22)
-    sass-rails (5.0.5)
+    sass-rails (5.0.6)
       railties (>= 4.0.0, < 6)
       sass (~> 3.1)
       sprockets (>= 2.8, < 4.0)
@@ -208,7 +208,7 @@ GEM
       ref
     thor (0.19.1)
     thread_safe (0.3.5)
-    tilt (2.0.5)
+    tilt (2.0.8)
     turbolinks (5.0.0)
       turbolinks-source (~> 5)
     turbolinks-source (5.0.0)
@@ -246,7 +246,7 @@ DEPENDENCIES
   rubocop (~> 0.39.0)
   rubocop-checkstyle_formatter (~> 0.2.0)
   rubycas-client!
-  sass-rails (~> 5.0)
+  sass-rails (~> 5.0, >= 5.0.6)
   sdoc (~> 0.4.0)
   simplecov
   simplecov-rcov