Restore .sbtopts file

I noticed that the sbt project has gotten much slower to compile on my
computer and that bumping the memory helped a lot. With 1GB, it seems to
take about 60-70 seconds for a clean compile and with 2GB it takes
between 35-40 seconds.

Note that this will break sbt loading on windows using the batch script.
This should be fixed in sbt 1.4.4:
https://github.com/sbt/sbt-launcher-package/pull/344.

.appveyor.yml

@@ -116,6 +116,7 @@ for:
     - SET PATH=C:\sbt\sbt\bin;%PATH%
     - SET JAVA_HOME=C:\jdk8
     - gu install native-image
+    - rm .sbtopts
 
   cache:
     - '%USERPROFILE%\.ivy2\cache'
@@ -148,6 +149,7 @@ for:
         }
     - SET PATH=C:\sbt\sbt\bin;%PATH%
     - SET SBT_OPTS=-Xmx4g -Dsbt.supershell=never -Dfile.encoding=UTF8
+    - rm .sbtopts
 
   cache:
     - '%USERPROFILE%\.ivy2\cache'

.sbtopts

@@ -0,0 +1 @@
+-J-Xmx2G