Fix quotes around Rscript source in .travis.yml

Fix line to run Rscript -e with quotes, since below fails

```
Rscript -e source(".ci_tests.R", echo=TRUE)
/home/travis/build.sh: eval: line 41: syntax error near unexpected token `('
/home/travis/build.sh: eval: line 41: `Rscript -e source(".ci_tests.R", echo=TRUE)'
```