Temp. fix for cc-test-reporter & SimpleCov 0.18

[Cruikshanks]

https://github.com/codeclimate/test-reporter/issues/418

On 28 Jan 2020 SimpleCov 0.18.0 was released. Though it's generating code coverage for us, its changed the JSON format of the /coverage/.resultset.json CodeClimate's cc-test-reporter appears to rely on.

Format before

```json { "RSpec": { "coverage": { "/path/to/my/file.rb": [ null, null, 1 ] }, "timestamp": 1580226312 } } ```

Format after

```json { "RSpec": { "coverage": { "/path/to/my/file.rb": { "lines": [ null, null, 1, null ], "branches": { } } }, "timestamp": 1580299988 } } ```

When cc-test-reporter is called in the after_script hook in Travis-CI we are seeing the following error

if [ "$TRAVIS_PULL_REQUEST" == "false" ]; then ./cc-test-reporter after-build --exit-code $TRAVIS_TEST_RESULT; fi
Error: json: cannot unmarshal object into Go struct field input.coverage of type []formatters.NullInt
Usage:
  cc-test-reporter after-build [flags]
Flags:
  -s, --batch-size int               batch size for source files (default 500)
  -e, --coverage-endpoint string     endpoint to upload coverage information to (default "https://api.codeclimate.com/v1/test_reports")
  -t, --coverage-input-type string   type of input source to use [clover, cobertura, coverage.py, excoveralls, gcov, gocov, jacoco, lcov, simplecov, xccov]
      --exit-code int                exit code of the test run
  -r, --id string                    reporter identifier (default "463a251defaae85271bff85ea8b1e58aa06cc9322157aa3ad7ee2a986aa13612")
      --insecure                     send coverage insecurely (without HTTPS)
  -p, --prefix string                the root directory where the coverage analysis was performed (default "/home/travis/build/DEFRA/waste-carriers-front-office")
Global Flags:
  -d, --debug   run in debug mode

As the CodeClimate integration is important to our CI process, we have chosen to temporarily set the version of SimpleCov as 0.17.1 until the issue is resolved.