Switch from sassc to sass gem

After setting up a new project with this template, both Netlify and
CircleCI were unable to bundle with sassc. Swapping for the regular sass
gem fixed the issue.

Gemfile

@@ -12,4 +12,4 @@ gem "middleman-minify-html", "~> 3.4"
 gem "neat", "~> 2.1"
 gem "rake", "~> 12.0"
 gem "redcarpet", "~> 3.4"
-gem "sassc", "~> 1.11"
+gem "sass", "~> 3.4"

Gemfile.lock

@@ -126,11 +126,7 @@ GEM
       http-cookie (>= 1.0.2, < 2.0)
       mime-types (>= 1.16, < 4.0)
       netrc (~> 0.8)
-    sass (3.4.24)
-    sassc (1.11.2)
-      bundler
-      ffi (~> 1.9.6)
-      sass (>= 3.3.0)
+    sass (3.4.25)
     servolux (0.12.0)
     thor (0.19.4)
     thread_safe (0.3.5)
@@ -157,10 +153,10 @@ DEPENDENCIES
   neat (~> 2.1)
   rake (~> 12.0)
   redcarpet (~> 3.4)
-  sassc (~> 1.11)
+  sass (~> 3.4)
 
 RUBY VERSION
    ruby 2.4.0p0
 
 BUNDLED WITH
-   1.15.1
+   1.15.3

README.md

@@ -19,7 +19,7 @@ This template comes with the following features and tools:
   a file.
 - [middleman-minify-html]: Minifies whitespace around HTML when the site is
   built.
-- [SassC]: CSS pre-processor.
+- [Sass]: CSS pre-processor.
 - [Bourbon]: Sass tool set.
 - [Neat]: Sass grid.
 - [Bitters]: Scaffold styles, variables and structure.
@@ -36,7 +36,7 @@ This template comes with the following features and tools:
   [middleman-autoprefixer]: https://github.com/middleman/middleman-autoprefixer
   [middleman-livereload]: https://github.com/middleman/middleman-livereload
   [middleman-minify-html]: https://github.com/middleman/middleman-minify-html
-  [SassC]: https://github.com/sass/sassc
+  [Sass]: https://github.com/sass/sass
   [Bourbon]: https://github.com/thoughtbot/bourbon
   [Neat]: https://github.com/thoughtbot/neat
   [Bitters]: https://github.com/thoughtbot/bitters