[fix][Travis] Fix Travis coveralls post-build step

**Why this change was necessary**
The `coveralls` post-success build step was erroring out due to the
now-wrong package being pip-installed.

**What this change does**
Modifies the setup step to install the correct coveralls package.

**Any side-effects?**
None noticed yet... Will update after running the build.

**Additional context/notes/links**

Resolves #143 - coveralls crashing in Travis - pip package install needs to be corrected

.travis.yml

@@ -22,7 +22,7 @@ install:
   - sudo install dcm2nii* /usr/bin/
   -
   - pip install .
-  - pip install python-coveralls
+  - pip install coveralls
 # command to run tests
 script:
   - echo "Download prelude"