test: change CI_TARGET reference to CI

Travis defines[1] $CI for its builds, whereas $CI_TARGET is
a Neovim-specific env var from 6483a198e4
that lost prominence in d2eb4a9346.

[1] https://docs.travis-ci.com/user/environment-variables/

.ci/script.sh

@@ -4,7 +4,7 @@ set -e
 set -o pipefail
 
 if [[ -n "${CI_TARGET}" ]]; then
-  make lint
+  make "${CI_TARGET}"
   exit 0
 fi
 

.travis.yml

@@ -58,7 +58,7 @@ env:
 matrix:
   include:
     - os: linux
-      env: CI_TARGET=clint
+      env: CI_TARGET=lint
     - os: linux
       compiler: gcc-5
     - os: linux

test/functional/ui/screen.lua

@@ -119,7 +119,7 @@ if os.getenv('VALGRIND') then
   default_screen_timeout = default_screen_timeout * 3
 end
 
-if os.getenv('CI_TARGET') then
+if os.getenv('CI') then
   default_screen_timeout = default_screen_timeout * 3
 end