grep: Make sure settings.gradle is in alphabetical order

exercises/settings.gradle

 include 'gigasecond'
 include 'go-counting'
 include 'grade-school'
+include 'grep'
 include 'hamming'
 include 'hexadecimal'
 include 'hello-world'
 include 'word-search'
 include 'wordy'
 include 'zipper'
-include 'grep'
-