3.1.0

• version: minor patch to 3.1.0 - @vmarchaud
• fix: log to logger when profiler isn't available + check for undefined #223 - @vmarchaud
• improv: edit configuration to use apm.version instead of pmx_version - @Eywek
• doc: add migration guides from 2.x to 3.x #220 - @vmarchaud
• feat: enable snapshot with node 11 - @vmarchaud
• tracing: add options to not censor spans labels - @vmarchaud
• doc: add documentation about express/koa middleware - @vmarchaud
• bug: fixes heap snapshot + enable profiling by default - @vmarchaud
• ci: fix slack notif step - @vmarchaud