Fix to output relative path correctly

[mtsmfm]

Currently filepath starts with / but in most case, it's a wrong path.

How to reproduce

git clone https://github.com/jest-community/jest-junit
cd jest-junit
yarn install
JEST_JUNIT_SUITE_NAME='{filepath}' yarn jest --env node integration-tests/reporter/__tests__/simple.test.js --reporters=default --reporters=$PWD/index.js

Before

<testsuites name="jest tests" tests="1" failures="0" time="0.336">
  <testsuite name="/integration-tests/reporter/__tests__/simple.test.js" errors="0" failures="0" skipped="0" timestamp="2018-12-18T10:54:15" time="0.216" tests="1">
    <testcase classname="foo should pass" name="foo should pass" time="0.003">
    </testcase>
  </testsuite>
</testsuites>

After

<testsuites name="jest tests" tests="1" failures="0" time="0.364">
  <testsuite name="integration-tests/reporter/__tests__/simple.test.js" errors="0" failures="0" skipped="0" timestamp="2018-12-18T10:55:11" time="0.241" tests="1">
    <testcase classname="foo should pass" name="foo should pass" time="0.003">
    </testcase>
  </testsuite>
</testsuites>

[mtsmfm]

@palmerj3 Can I ask you to review?

[palmerj3]

@mtsmfm the change itself looks good and thanks for taking the time to contribute!

Before I merge could you please just provide a rationale for this change? What problem(s) is it solving on your end?

Thanks!