Change Spark version installed by R library to 3.4.0

lib/R/pom.xml

@@ -59,10 +59,11 @@
     <pathling.Rapi.version.qualifier/>
     <pathling.Rapi.version>${pathling.Rapi.version.base}${pathling.Rapi.version.qualifier}</pathling.Rapi.version>
     <!-- 
-      This needs to maintained as the most recent patch version of Spark, otherwise we are forced to
-      download from the archive site which is very unreliable and frequently breaks the build. 
+      This needs to maintained as the most recent version of Spark that is in the versions manifest
+      of the sparklyr version that we depend upon that is on the archive site. The reason we can't 
+      use the latest from the CDN is that the files drop off the CDN when a new version is released.
     -->
-    <pathling.Rapi.sparkVersion>${pathling.sparkVersion}</pathling.Rapi.sparkVersion>
+    <pathling.Rapi.sparkVersion>3.4.0</pathling.Rapi.sparkVersion>
     <pathling.Rapi.scalaVersion>${pathling.scalaVersion}</pathling.Rapi.scalaVersion>
     <pathling.Rapi.hadoopMajorVersion>${pathling.hadoopMajorVersion}</pathling.Rapi.hadoopMajorVersion>
     <pathling.Rapi.hadoopVersion>${pathling.hadoopVersion}</pathling.Rapi.hadoopVersion>