Upgrade mail gem to drop dependency on mime-types.

Boot Memory Usage

```
Total allocated: 225695379 bytes (1990663 objects)
Total retained:  32795116 bytes (291607 objects)
```

```
Total allocated: 218342892 bytes (1897350 objects)
Total retained:  30123426 bytes (261198 objects)
```

.gitattributes

@@ -1,6 +1,9 @@
 # Set default behaviour, in case users don't have core.autocrlf set.
 * text=auto
 
+# Treat email fixtures as binary files so CRLF are not converted to LF.
+*.eml binary
+
 # Explicitly declare text files we want to always be normalized and converted
 # to native line endings on checkout.
 *.yml text

Gemfile

@@ -26,7 +26,6 @@ else
 end
 
 gem 'mail'
-gem 'mime-types', require: 'mime/types/columnar'
 gem 'mini_mime'
 gem 'mini_suffix'
 

Gemfile.lock

@@ -162,16 +162,13 @@ GEM
       crass (~> 1.0.2)
       nokogiri (>= 1.5.9)
     lru_redux (1.1.0)
-    mail (2.6.6)
-      mime-types (>= 1.16, < 4)
+    mail (2.7.0)
+      mini_mime (>= 0.1.1)
     memory_profiler (0.9.10)
     message_bus (2.1.2)
       rack (>= 1.1.3)
     metaclass (0.0.4)
     method_source (0.8.2)
-    mime-types (3.1)
-      mime-types-data (~> 3.2015)
-    mime-types-data (3.2016.0521)
     mini_mime (0.1.3)
     mini_portile2 (2.3.0)
     mini_racer (0.1.15)
@@ -442,7 +439,6 @@ DEPENDENCIES
   mail
   memory_profiler
   message_bus
-  mime-types
   mini_mime
   mini_racer
   mini_suffix