Add code coverage badge

[fractalwrench]

Add badge showing coveralls coverage

[coveralls]

Pull Request Test Coverage Report for Build 1340

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• 18 unchanged lines in 3 files lost coverage.
• Overall coverage remained the same at 74.291%

---

Files with Coverage Reduction	New Missed Lines	%
sdk/src/main/java/com/bugsnag/android/DeviceData.java	2	76.69%
sdk/src/main/java/com/bugsnag/android/Client.java	3	62.56%
sdk/src/main/java/com/bugsnag/android/SessionTracker.java	13	57.89%

Totals
Change from base Build 1338:	0.0%
Covered Lines:	1861
Relevant Lines:	2505

---

💛 - Coveralls