fix(install): use prepublish script rather than install

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please note: I wasn't able to run the tests due to a missing devDependency, but I've verified that with this change allows me to install sauce-connect-launcher@1.7.0 even while depending on lodash directly.  commit message copied below
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scripts/install.js requires lodash (and other modules), but if they are
direct dependencies of an application then they may not be available
when the `install` step runs in npm.

Convert from running the script as an `install` script to a `prepublish`
script so that other application dependencies are available at the time
it runs.

Closes #37