fix code coverage (#14)

Author: Pietro Vertechi

.travis.yml

@@ -30,6 +30,6 @@ git:
 #  - julia -e 'Pkg.clone(pwd()); Pkg.build("StructArrays"); Pkg.test("StructArrays"; coverage=true)'
 after_success:
   # push coverage results to Coveralls
-  - julia -e 'cd(Pkg.dir("StructArrays")); Pkg.add("Coverage"); using Coverage; Coveralls.submit(Coveralls.process_folder())'
+  - julia -e 'import Pkg; cd(Pkg.dir("StructArrays")); Pkg.add("Coverage"); using Coverage; Coveralls.submit(Coveralls.process_folder())'
   # push coverage results to Codecov
-  - julia -e 'cd(Pkg.dir("StructArrays")); Pkg.add("Coverage"); using Coverage; Codecov.submit(Codecov.process_folder())'
+  - julia -e 'import Pkg; cd(Pkg.dir("StructArrays")); Pkg.add("Coverage"); using Coverage; Codecov.submit(Codecov.process_folder())'