Add detection for Hack files with ".php" file extension

Based on top of PR#1447. Adds a simple heuristic check for Hack files vs PHP files (`<?hh` vs other `<?`).

Tested by verifying that the Hack example site was detected as 100% Hack and that Laravel was detected as 100% PHP. (Without the heuristic, Laravel gets detected as about 50% Hack, just by randomness in the classifier since PHP and Hack are very hard to distinguish unless you actually parse the file and look for specific language features.)

lib/linguist/heuristics.rb

@@ -25,6 +25,9 @@ module Linguist
         if languages.all? { |l| ["Common Lisp", "OpenCL"].include?(l) }
           result = disambiguate_cl(data, languages)
         end
+        if languages.all? { |l| ["Hack", "PHP"].include?(l) }
+          result = disambiguate_hack(data, languages)
+        end
         return result
       end
     end
@@ -88,6 +91,13 @@ module Linguist
       matches
     end
 
+    def self.disambiguate_hack(data, languages)
+      matches = []
+      matches << Language["Hack"] if data.include?("<?hh")
+      matches << Language["PHP"] if /<?[^h]/.match(data)
+      matches
+    end
+
     def self.active?
       !!ACTIVE
     end