Change BQ mode to append instead of truncate

[ravwojdyla]

It appears that truncate mode is not supported for unbounded
PCollection.

Error message:

[ERROR] Failed to execute goal
org.codehaus.mojo:exec-maven-plugin:1.1:java (default-cli) on
project
google-cloud-dataflow-java-examples-all: An exception occured while
executing th
e Java class. null: InvocationTargetException:
WriteDisposition.WRITE_TRUNCATE is not supported for unbounded
PCollections or when using tablespec functions. -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with
the
-e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug
logging.
[ERROR]
[ERROR] For more information about the errors and possible
solutions,
please read the following articles:
[ERROR] [Help 1]
http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException

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[ravwojdyla]

RePR from GoogleCloudPlatform/DataflowSDK-examples#2
CLA signed as corporation.

[dhalperi]

CLA confirmed Spotify.

You can prevent this in the future by updating your git config user.email to be your corporate address.

[dhalperi]

Rafal, sorry for the delay in merging this. We're working out a problem with our Travis build, then we'll get on it :).

[davorbonaci]

Merged. Thanks!