Add support for clojure 1.9

[davidevernizzi]

I have removed the .incubator so that clostache can be used with clojure 1.9

[kumarshantanu]

Making a note that this PR will remove support for Clojure 1.8 and earlier.

[jgrodziski]

Or bump clojure.core.incubator to 0.1.4 where the seqable? function is correctly deprecated and the exclude is properly done so we don't get any warning?

[rajkrish]

I am getting the warnings with Clojars version of Clostache 1.4.0, while using rebel-readline REPL -

Downloading: de/ubercode/clostache/clostache/1.4.0/clostache-1.4.0.pom from clojars
Downloading: org/clojure/core.incubator/0.1.2/core.incubator-0.1.2.pom from central
....
....
....
WARNING: seqable? already refers to: #'clojure.core/seqable? in namespace: clojure.core.incubator, being replaced by: #'clojure.core.incubator/seqable?
WARNING: seqable? already refers to: #'clojure.core/seqable? in namespace: clostache.parser, being replaced by: #'clojure.core.incubator/seqable?

I would appreciate if someone could publish an updated version of the fix that addresses the warnings, to Clojars.
Thank you.