Try using --substituters for the ghcjs build

.travis.yml

@@ -27,7 +27,7 @@ env:
     - GHCVERSION=ghc822 STRICT=true  TRACING=true  PROFILING=true
     - GHCVERSION=ghc842 STRICT=false TRACING=false PROFILING=false
     - GHCVERSION=ghc842 STRICT=false TRACING=true  PROFILING=true
-    # - GHCVERSION=ghcjs
+    - GHCVERSION=ghcjs
 
 matrix:
   allow_failures:

build.sh

@@ -9,7 +9,7 @@ TRACING=${TRACING:-false}
 PROFILING=${PROFILING:-false}
 
 if [ "$GHCVERSION" = "ghcjs" ]; then
-    NIX_CONF_DIR=$PWD/ghcjs nix-build ghcjs
+    nix-build --substituters 'https://nixcache.reflex-frp.org?trusted=1' ghcjs
 else
     nix-build                                   \
         --argstr compiler $GHCVERSION           \