Remove Nimble beta qualifier (#972)

It seems to me that Nimble is not beta software at this point. It's widely used, fairly stable, and mostly does what is needed. Especially with the addition of lock files, I don't believe the beta qualifier fits. It's certainly not worse than the Python or Go-lang package managers, and arguably better in many ways. 

The reason for changing this is that it gives a bit of a "Nim's package manger is beta? Should I really be sure about this language... ".

readme.markdown

@@ -1,6 +1,6 @@
 # Nimble
 
-Nimble is a *beta*-grade *package manager* for the [Nim programming
+Nimble is the default *package manager* for the [Nim programming
 language](https://nim-lang.org).
 
 Interested in learning **how to create a package**? Skip directly to that section