Make Nimble compatible with the upcoming compiler API changes (#500)

* Make Nimble compatible with the upcoming compiler API changes

* make travis use nim-0.18.0

* make Nimble compile with older compiler versions

* attempt to make tests green with Nim v0.18

.travis.yml

@@ -6,16 +6,16 @@ language: c
 
 install:
   - |
-      wget https://nim-lang.org/download/nim-0.17.2.tar.xz
-      tar -xf nim-0.17.2.tar.xz
-      cd nim-0.17.2
+      wget https://nim-lang.org/download/nim-0.18.0.tar.xz
+      tar -xf nim-0.18.0.tar.xz
+      cd nim-0.18.0
       sh build.sh
       cd ..
 
 before_script:
   - set -e
   - set -x
-  - export PATH=`pwd`/nim-0.17.2/bin:$PATH
+  - export PATH=`pwd`/nim-0.18.0/bin:$PATH
 
 script:
   - cd tests