Remove zip dependency and replace with unzip equivalent (#354)

diff --git a/Dockerfile b/Dockerfile
index 0502845..90a733d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,6 +1,6 @@
 FROM openjdk:8-jdk
 
-RUN apt-get update && apt-get install -y git curl zip && rm -rf /var/lib/apt/lists/*
+RUN apt-get update && apt-get install -y git curl && rm -rf /var/lib/apt/lists/*
 
 ENV JENKINS_HOME /var/jenkins_home
 ENV JENKINS_SLAVE_AGENT_PORT 50000
diff --git a/install-plugins.sh b/install-plugins.sh
index 8b8daee..25c12f4 100755
--- a/install-plugins.sh
+++ b/install-plugins.sh
@@ -76,7 +76,7 @@
     plugin="$1"
     jpi="$(getArchiveFilename "$plugin")"
 
-    zip -T "$jpi" >/dev/null
+    unzip -t -qq "$jpi" >/dev/null
     return $?
 }