Fix travis CI for NPM

Currently it's complaining about missing dependency of headless chrome:

    Running "qunit:ThriftDeepConstructor" (qunit) task
    >> There was an error with headless chrome
    Fatal error: Failed to launch chrome!
    /thrift/src/lib/js/node_modules/puppeteer/.local-chromium/linux-609904/chrome-linux/chrome: error while loading shared libraries: libXtst.so.6: cannot open shared object file: No such file or directory
    TROUBLESHOOTING: https://github.com/GoogleChrome/puppeteer/blob/master/docs/troubleshooting.md
diff --git a/build/docker/ubuntu-bionic/Dockerfile b/build/docker/ubuntu-bionic/Dockerfile
index 98f2607..291aa5d 100644
--- a/build/docker/ubuntu-bionic/Dockerfile
+++ b/build/docker/ubuntu-bionic/Dockerfile
@@ -179,7 +179,8 @@
 # Test dependencies for running puppeteer
 RUN apt-get install -y --no-install-recommends \
 `# JS dependencies` \
-      libxss1
+      libxss1 \
+      libxtst6
 
 RUN apt-get install -y --no-install-recommends \
 `# OCaml dependencies` \
diff --git a/build/docker/ubuntu-disco/Dockerfile b/build/docker/ubuntu-disco/Dockerfile
index 2c3166f..9775fbb 100644
--- a/build/docker/ubuntu-disco/Dockerfile
+++ b/build/docker/ubuntu-disco/Dockerfile
@@ -179,7 +179,8 @@
 # Test dependencies for running puppeteer
 RUN apt-get install -y --no-install-recommends \
 `# JS dependencies` \
-      libxss1
+      libxss1 \
+      libxtst6
 
 # does not work on disco?
 # RUN apt-get install -y --no-install-recommends \
diff --git a/build/docker/ubuntu-xenial/Dockerfile b/build/docker/ubuntu-xenial/Dockerfile
index c0f42c7..62da2c4 100644
--- a/build/docker/ubuntu-xenial/Dockerfile
+++ b/build/docker/ubuntu-xenial/Dockerfile
@@ -170,6 +170,7 @@
 RUN apt-get install -y --no-install-recommends \
 `# JS dependencies` \
       libxss1 \
+      libxtst6 \
       libatk-bridge2.0-0 \
       libgtk-3-0