Fix space.

Author: Francois Best

.travis.yml

@@ -74,7 +74,7 @@ script:
     # Build current example
     - |
         if [ ! "${BUILD_UNIT_TESTS}" ]; then
-            if [ "${REQUIRES_USB}"]; then
+            if [ "${REQUIRES_USB}" ]; then
                 platformio ci --lib="." --lib=external/midi-usb --board="due" --board="zero" --board="leonardo"
             else
                 platformio ci --lib="." --board=uno --board="due" --board="zero" --board="leonardo"