Paket magic mode fails on Travis-CI with Win32Exception

Description

https://travis-ci.org/sideeffffect/FSharpx.Collections/builds/261426969

$ ./build.sh All
Running paket failed with: System.ComponentModel.Win32Exception: ApplicationName='/tmp/paket_F9967CACF72243EFF862145CF0582A9916D34B1C786AAE38D5C4CD10F307A27F.exe', CommandLine='--from-bootstrapper restore', CurrentDirectory='', Native error= Cannot find the specified file
  at System.Diagnostics.Process.Start_noshell (System.Diagnostics.ProcessStartInfo startInfo, System.Diagnostics.Process process) [0x00000] in <filename unknown>:0 
  at System.Diagnostics.Process.Start_common (System.Diagnostics.ProcessStartInfo startInfo, System.Diagnostics.Process process) [0x00000] in <filename unknown>:0 
  at System.Diagnostics.Process.Start () [0x00000] in <filename unknown>:0 
  at (wrapper remoting-invoke-with-check) System.Diagnostics.Process:Start ()
  at Paket.Bootstrapper.PaketRunner.Run (System.String program, IEnumerable`1 arguments) [0x00000] in <filename unknown>:0 
  at Paket.Bootstrapper.Program.OnSuccessfulDownload (Paket.Bootstrapper.BootstrapperOptions options) [0x00000] in <filename unknown>:0 
Cannot open assembly 'packages/FAKE/tools/FAKE.exe': No such file or directory.

Repro steps

run Travis-CI of e.g. FSharpx.Collections

Expected behavior

Paket should restore packages

Actual behavior

but instead it crashes

Known workarounds

use paket.bootstrapper.exe (see https://github.com/fsprojects/FSharpx.Collections/pull/78)

Start by reproducing the failure through build.sh All on Travis-CI, using the FSharpx.Collections example and the reported paket magic mode path. Compare that behavior with the paket.bootstrapper.exe workaround and verify that the fix restores packages so packages/FAKE/tools/FAKE.exe can be opened.