Fix emsdk on python3 - the values() of a dict is a view there, not a list (#241)

Also add both python2 and 3 testing on CI here.

Fixes #240.

Dockerfile

diff --git a/Dockerfile b/Dockerfile
index 910a730..03a9fd4 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -8,7 +8,7 @@
 RUN cd /root/ \
  && echo "int main() {}" > hello_world.cpp \
  && apt-get update \
- && apt-get install -y python cmake build-essential openjdk-9-jre-headless \
+ && apt-get install -y python python3 cmake build-essential openjdk-9-jre-headless \
  && /root/emsdk/emsdk update-tags \
  && echo "test latest" \
  && /root/emsdk/emsdk install latest \
@@ -30,12 +30,12 @@
  && emcc hello_world.cpp -s WASM=0 \
  && emcc --clear-cache \
  && echo "test latest-releases-upstream" \
- && /root/emsdk/emsdk install latest-releases-upstream \
+ && python2 /root/emsdk/emsdk install latest-releases-upstream \
  && /root/emsdk/emsdk activate latest-releases-upstream \
  && source /root/emsdk/emsdk_env.sh --build=Release \
  && emcc hello_world.cpp \
  && echo "test latest-releases-fastcomp" \
- && /root/emsdk/emsdk install latest-releases-fastcomp \
+ && python3 /root/emsdk/emsdk install latest-releases-fastcomp \
  && /root/emsdk/emsdk activate latest-releases-fastcomp \
  && source /root/emsdk/emsdk_env.sh --build=Release \
  && emcc hello_world.cpp \

emsdk

diff --git a/emsdk b/emsdk
index c60d09f..3397f2b 100755
--- a/emsdk
+++ b/emsdk
@@ -1875,7 +1875,7 @@
 # Get a list of tags for emscripten-releases.
 def load_releases_tags():
   info = load_releases_info()
-  return info.values()
+  return list(info.values())
 
 
 def is_string(s):