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

@@ -1,11 +1,14 @@
 # Set default behaviour, in case users don't have core.autocrlf set.
 * text=auto
 
-# Explicitly declare text files we want to always be normalized and converted 
+# 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
 
-# Custom for Visual Studio, very unlikely, but lets keep it 
+# Custom for Visual Studio, very unlikely, but lets keep it
 *.cs     diff=csharp
 *.sln    merge=union
 *.csproj merge=union