Update links referencing default branch

Author: awvwgk

README.md

@@ -24,11 +24,11 @@ Further updates are planned...
 
 ### Documentation
 
- * The API documentation for the current ```master``` branch can be found [here](https://fortran-lang.github.io/minpack/).  This is generated by processing the source files with [FORD](https://github.com/Fortran-FOSS-Programmers/ford).
+ * The API documentation for the latest default branch can be found [here](https://fortran-lang.github.io/minpack/).  This is generated by processing the source files with [FORD](https://github.com/Fortran-FOSS-Programmers/ford).
 
 ### License
 
-The Minpack source code and related files and documentation are distributed under a permissive free software [license](https://github.com/fortran-lang/minpack/blob/master/LICENSE.txt) (BSD-style).
+The Minpack source code and related files and documentation are distributed under a permissive free software [license](https://github.com/fortran-lang/minpack/blob/HEAD/LICENSE.txt) (BSD-style).
 
 ### See also
 
@@ -40,4 +40,4 @@ The Minpack source code and related files and documentation are distributed unde
   * J. J. Moré, D. C. Sorensen, K. E. Hillstrom, and B. S. Garbow, The MINPACK Project, in Sources and Development of Mathematical Software, W. J. Cowell, ed., Prentice-Hall, pages 88-111, 1984.
   * M. J. D. Powell, A Hybrid Method for Nonlinear Equations. Numerical Methods for Nonlinear Algebraic Equations, P. Rabinowitz, editor. Gordon and Breach, 1970.
   * Jorge J. More, The Levenberg-Marquardt Algorithm, Implementation and Theory. Numerical Analysis, G. A. Watson, editor. Lecture Notes in Mathematics 630, Springer-Verlag, 1977.
-  * [MINPACK-2](https://ftp.mcs.anl.gov/pub/MINPACK-2/)
+  * [MINPACK-2](https://ftp.mcs.anl.gov/pub/MINPACK-2/)