.appveyor.yml: Disable build step

Otherwise AppVeyor fails with 
`Specify a project or solution file. The directory does not contain a project or solution file.`
https://ci.appveyor.com/project/spderosso/gitless/build/1.0.3/job/e54caey8a0b8knx3

Author: techtonik

.appveyor.yml

@@ -25,6 +25,8 @@ environment:
 init:
   - cmd: '%PYTHON% -m pip install -U nose wheel pygit2'
 
+build: off
+
 before_test:
   - cmd: git config --global user.name "appveyor-test"
   - cmd: git config --global user.email "appveyor@test.com"