fix errors due to use of unportable == operator

The `==` operator is not defined by POSIX and many test(1) (also
spelled `[`) implementations do not support it.  Replace uses of
`==` with the POSIX-conformant `=` equality operator.

nvm.sh

@@ -351,7 +351,7 @@ nvm() {
     ;;
     "uninstall" )
       [ $# -ne 2 ] && nvm help && return
-      if [ $2 == `nvm_version` ]; then
+      if [ "$2" = `nvm_version` ]; then
         echo "nvm: Cannot uninstall currently-active node version, $2."
         return 1
       fi