Fixing error: NoMethodError - undefined method `gsub' for nil:NilClass

I was getting an error in lib/syntax_highlighter.rb where 
```ruby
SyntaxHighlighter.pygmentize(file_type, blob.data)
```
was returning nil and thus throwing an error in
```ruby
def self.global_highlighting(pygmentized_text)
```
when it tried to call the gsub method on the nil object.

I'm primarily a PHP developer so please feel free to school me on Ruby methodologies that I may have screwed up.