Removed Boost dependency in language distinction.

As Boost isn’t used in the rest of this project anymore, this commit
removes the unnecessary dependency to boost::bimap just for language
detection, and replaces it with a simple std::map instead.

include/plasp/Language.h

@@ -23,7 +23,6 @@ class Language
 			SAS
 		};
 
-		static std::string toString(Type language);
 		static Language::Type fromString(const std::string &languageName);
 
 	public: