Update Codecov instructions

The current Codecov bash script automatically searches in the DerivedData directory for code coverage files (both gcov and profdata files). In order to prevent the codecov script to automatically search for coverage data and generate reports, we disable report generation (-X coveragepy -X gcov -X xcode) and tell it to upload the report generated by slather (-f path/to/xml_report/cobertura.xml).

README.md

@@ -91,7 +91,7 @@ before_install: rvm use $RVM_RUBY_VERSION
 install: bundle install --without=documentation --path ../travis_bundle_dir
 after_success: 
  - slather
- - bash <(curl -s https://codecov.io/bash) -f path/to/xml_report/cobertura.xml
+ - bash <(curl -s https://codecov.io/bash) -f path/to/xml_report/cobertura.xml -X coveragepy -X gcov -X xcode
 ```
 
 ```yml
@@ -100,7 +100,7 @@ after_success:
 test:
  post:
  - bundle exec slather
- - bash <(curl -s https://codecov.io/bash)
+ - bash <(curl -s https://codecov.io/bash) -f path/to/xml_report/cobertura.xml -X coveragepy -X gcov -X xcode
 ```
 
 > Private repo? Add `-t :uuid-repo-token` to the codecov uploader. Read more about uploading report to Codecov [here](https://github.com/codecov/codecov-bash)