define consistent binary filename in promu configuration file

[chriwill]

Maintaining a fork with a different folder name breaks the travis build/test.

Sympton:

>> formatting code
>> vetting code
>> running staticcheck
>> building binaries
 >   prometheus-node-exporter
./ttar -C collector/fixtures -x -f collector/fixtures/sys.ttar
touch collector/fixtures/sys/.unpacked
>> running tests
ok  	github.com/prometheus/node_exporter	1.029s
ok  	github.com/prometheus/node_exporter/collector	1.068s
?   	github.com/prometheus/node_exporter/collector/ganglia	[no test files]
>> running end-to-end tests
./end-to-end-test.sh
./node_exporter not found. Consider running `go build` first.
make: *** [test-e2e] Error 1
The command "make" exited with 2.
Done. Your build exited with 1.

This pull request defines a consistent binary filename in the promu configuration file.

After applying this pull request:

>> building binaries
 >   node_exporter
./ttar -C collector/fixtures -x -f collector/fixtures/sys.ttar
touch collector/fixtures/sys/.unpacked
>> running tests
ok  	github.com/prometheus/node_exporter	1.508s
ok  	github.com/prometheus/node_exporter/collector	1.089s
?   	github.com/prometheus/node_exporter/collector/ganglia	[no test files]
>> running end-to-end tests
./end-to-end-test.sh
The command "make" exited with 0.
Done. Your build exited with 0.

Best Regards

[SuperQ]

LGTM

[discordianfish]

LGTM, thanks!