compatibility: Explicitly specify template type to support older compilers

This makes curret development branch buildable on Ubuntu 18 / Centos 7 /
SUSE 15.

src/commands/CmdContext.cpp

@@ -123,7 +123,7 @@ std::vector <std::string> CmdContext::getContexts ()
         contexts.insert (suffix);
     }
 
-  return std::vector (contexts.begin (), contexts.end ());
+  return std::vector <std::string> (contexts.begin (), contexts.end ());
 }
 
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