4.2.0

• version: minor bump to 4.2.0 - @Eywek
• Fixed module path in PG Unit test case to pick root directory - @Anuj Verma
• Modified pg path in opencensus plugin - @Anuj Verma
• feat: implement retrieving of stack context from errors #216 - @vmarchaud
• ci: add pull request trigger - @Eywek
• tracing: fix undefined callback crash with redis - @vmarchaud
• tests: remove some assert because context propagation bug in node 6 - @vmarchaud
• tracing: make createSpanWithNet optional - @vmarchaud
• ci: add net tracing test - @vmarchaud
• tracing: add plugin for net server - @vmarchaud
• tests: fix test with packet channel assert - @vmarchaud
• feat: implement dependencies collection at startup #238 - @vmarchaud
• tests: fix tracing integration test with express - @vmarchaud
• tracing: enable express plugin by default - @vmarchaud
• tracing: distingish route/router from middlewares - @vmarchaud
• tests: fix new node types - @vmarchaud
• tracing: add express plugin - @vmarchaud