Update README.arm.md

I needed to `apt-get install m4` as well.
jonathanvdc · May 27, 2015 · c78161d · c78161d
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commit c78161d
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diff --git a/README.arm.md b/README.arm.md
@@ -47,7 +47,7 @@ override USE_SYSTEM_ARPACK=1
 The following command will install all the necessary libraries on Ubuntu.
 
 ````
-sudo apt-get install libblas3gf liblapack3gf libfftw3-dev libgmp3-dev libmpfr-dev libblas-dev liblapack-dev cmake gcc-4.8 g++-4.8 gfortran libgfortran3
+sudo apt-get install libblas3gf liblapack3gf libfftw3-dev libgmp3-dev libmpfr-dev libblas-dev liblapack-dev cmake gcc-4.8 g++-4.8 gfortran libgfortran3 m4
 ````
 
 # ARM specific build problems