Version bump for release 0.1.8

cxxopts 2.1.0 has a bug preventing it from being used with standard main
signatures. This updates cxxopts to the commit after release 2.1.0,
where this issue was addressed.

CHANGELOG.md

@@ -1,6 +1,6 @@
 # Change Log
 
-## 0.1.8 RC 3 (2018-04-12)
+## 0.1.8 (2018-04-20)
 
 ### Features
 

app/main.cpp

@@ -70,7 +70,7 @@ int main(int argc, char **argv)
 
 	if (version)
 	{
-		std::cout << "anthem version 0.1.8-rc.3" << std::endl;
+		std::cout << "anthem version 0.1.8" << std::endl;
 		return EXIT_SUCCESS;
 	}