Travis CI: remove stray $ in .travis.yml

Fixes: 1d0ba96 ("Add GLFW-installed-but-using-our-GLFW-anyway configuration")

.travis.yml

@@ -14,7 +14,7 @@ matrix:
       env: ARCH=i386
       sudo: required
     - os: linux
-      env: ARCH=amd64 INSTALL_GLFW=YES USE_EXTERNAL_GLFW=NO
+      env: ARCH=amd64 INSTALL_GLFW=YES USE_EXTERNAL_GLFW=OFF
       sudo: required
     - os: linux
       env: ARCH=amd64 INSTALL_GLFW=YES RELEASE=NO
@@ -104,7 +104,7 @@ script:
       git checkout 1.12 && ./autogen.sh --prefix=/usr && make && sudo make install;
       popd;
     fi
-  - $RUNNER cmake $CMAKE_ARCH_ARGS -DMACOS_FATLIB=ON -DSTATIC=ON -DSHARED=ON -DBUILD_EXAMPLES=ON -DBUILD_GAMES=ON -DUSE_EXTERNAL_GLFW=$USE_EXTERNAL_GLFW$ -DUSE_WAYLAND=${USE_WAYLAND} ..
+  - $RUNNER cmake $CMAKE_ARCH_ARGS -DMACOS_FATLIB=ON -DSTATIC=ON -DSHARED=ON -DBUILD_EXAMPLES=ON -DBUILD_GAMES=ON -DUSE_EXTERNAL_GLFW=$USE_EXTERNAL_GLFW -DUSE_WAYLAND=${USE_WAYLAND} ..
   - $RUNNER make VERBOSE=1
   - if [ "$RELEASE" != "NO" ]; then $RUNNER make package; fi;
   - sudo $RUNNER make install