Bump dependency versions, support node 8+ (disabled node 6-)

- Update all dependencies
- "clarify" and "stack" now use the same version of "stack-chain",
  this fixes the error "Conflicting version of stack-chain found"
- trace and clarify are now activated only for node 8 and higher

The last item could be seen as breaking change, but I am not treating
it as such, because

- the change does not break the normal execution of the program
- the whole library is for debugging purposes (humans). It does not
  have an API.

index.js

@@ -7,9 +7,9 @@
 
 'use strict'
 
-var versionStr = process.version.match(/v(\d+).*/)[1]
-var majorVersion = Number(versionStr)
-if (majorVersion >= 4 && majorVersion < 8) {
+const versionStr = process.version.match(/v(\d+).*/)[1]
+const majorVersion = Number(versionStr)
+if (majorVersion >= 8) {
   require('trace')
   require('clarify')
   console.log('Trace and clarify activated')