fixes the travis build for NIM_COMPILE_TO_CPP

.travis.yml

@@ -42,7 +42,7 @@ before_script:
   - export PATH=$(pwd)/bin${PATH:+:$PATH}
 script:
   - nim c koch
-  - ./koch boot
+  - env NIM_COMPILE_TO_CPP=false ./koch boot
   - ./koch boot -d:release
   - ./koch nimble
   - nim e tests/test_nimscript.nims