Optimise build times by specifying only the generators we use in R.swift

From the [R.swift documentation](https://github.com/mac-cain13/R.swift/blob/master/Documentation/Ignoring.md#only-run-specific-generators-exclude-rsomething)
By default, R.swift runs all generators, for images, nibs, strings and many more. In some situations you may not want to generate R structs for all these types. You can choose to run only certain generators by adding a flag like this: --generators image,string to the call to the [Build Phase](https://github.com/mac-cain13/R.swift/blob/master/Documentation/Images/BuildPhaseExample.png)
```
These are the available generators:

image
string
color
file
font
nib
segue
storyboard
reuseIdentifier
entitlements
info
id
```
I believe we only use image, string, color, file, font. We should optimise this in order to reduce build times.